Screw type implants are implants having outer surfaces being threaded and which are used as anchoring members for different prostheses, such as dental and orthopaedic prostheses. To this end, this type of implant is screwed into a bore hole arranged in the bone tissue of a bone tissue structure at a site where a prosthesis is required. The bore hole may be formed to a shape generally corresponding to the shape of the implant, although slightly smaller in size. These implants may be provided with self-cutting edges, so as to cut one or more internal threads in the inner wall of the bore hole during the screwing in of the implant. If there are no self-cutting edges, the bore must be internally threaded before insertion of the implant.
Bone tissue has two components, cancellous bone tissue and cortical bone tissue. The major part of a bone is normally built up of the cancellous bone tissue, which is a relatively soft tissue in the interior of the bone. The cortical bone tissue is harder and normally forms a relatively thin layer surrounding the cancellous bone. Thus, in their final position, screw implants of the type described would typically be in contact with cancellous bone tissue along a larger part of its length, and with cortical bone tissue only at a shorter portion at one end of the implant.
When a screw type implant is in anchored position in the bone tissue, a superstructure for carrying a prosthetic part may be secured to the implant. In the case when a screw implant will be used to secure a dental prosthesis, the superstructure will typically comprise an abutment or transmucosal component, which engages the implant to bridge the gingiva overlaying the maxilla or mandible at the implant site. The prosthetic part, e.g. a crown, a bridge or a denture is then secured to the abutment. The implant could also be formed integrally with a superstructure, such as a transgingival component, on which for example a crown is directly secured.
A problem occurring when using many prior art screw type implants is referred to as the bone resorption problem. Bone resorption is a term used for a process in which, once an implant is installed in the bone tissue, the bone surrounding the implant tends to degenerate. This is highly undesired, since a diminished amount of bone surrounding the implant will lead to diminished stability and sometimes result in failure of the prosthesis. This is particularly the case because bone resorption primarily occurs in the cortical bone, which, as mentioned above, is the hardest part of the bone. Once bone resorption is a fact, secondary problems may also appear. Such secondary problems, particularly related to dental implants, are for example deposition of plaque, resulting in inflammation in the gingival tissue surrounding the implant, or down-growth of gingival tissue along the exposed end of the implant. Also, the aesthetic appeal of the implant is undermined by bone tissue resorption, which is an important drawback in particular when the implant is intended for dental applications since dental prosthesis form part of the field of cosmetic surgery.
The biological causes of bone resorption are not yet completely understood. According to the inventors' belief, it is however important to ensure proper loading of the implant, since both mechanical over-stimulation and under-stimulation of bone tissue have been seen to cause bone resorption. Prior work by the inventors has been directed towards the issue of developing an implant that transmit the axial loading applied thereupon in an appropriate way to bone. During this work, it has been found to be relevant that the loading is distributed evenly to the adjacent bone tissue, meaning that large stress concentrations or peaks are avoided.
WO 00/03657 (Astra Aktiebolag; Hansson) discloses a prior art screw type implant having a cylindrical shaft, which is adapted in use to be embedded in bone tissue, and which has an outer surface provided with a circumferentially-oriented roughness. The circumferentially-oriented roughness has first and second axial sections with each section comprising a series of circumferentially-oriented peaks which have a crest and which are axially spaced apart by troughs. The axial spacing between the crests of adjacent peaks in the first axial section is less than the axial spacing between the crests of adjacent peaks in the second axial section. The first and second axial sections of circumferentially-oriented roughness are adapted to provide the same or substantially the same pitch. The threads constituting the circumferentially-oriented roughness are self-tapping. Further, the implant as described is to be substantially completely submerged into the bone tissue and the top of the implant is flush with the outer surface of the bone into which it is inserted.
In the above-mentioned implant, the first and second sections of circumferentially-oriented roughness improve the ability of the implant to transmit load evenly to the bone tissue in order to inhibit marginal bone resorption.
A disadvantage with the above-mentioned implant might be revealed when installing it, as is common, into a bore hole. The size of the bore hole should be adapted so that the implant can be screwed into the hole, using its self-tapping cutting edges to cut a thread in the walls of the bore hole. The threads created in that way are useful for holding the implant in place, thus increasing its stability and promoting the healing process. Unfortunately, there is a risk that the surgeon, when having screwed the implant into a position where it has reached the bottom of the bore hole, happens to continue screwing the implant. Naturally, the implant cannot be screwed further into the bone from this position. Instead, the extra screwing merely rotates the implant in the bore hole, while it remains at the same depth, which rotation causes destruction of the previously cut internal threads in the bore hole. Without the internal threads, the stability of the implant is impaired. Less stability leads to longer healing time for the implant, and an increased risk for secondary problems. Such a secondary problem, being particularly problematic in dental applications is the formation of unwanted soft connective tissue around the implant.
U.S. Pat. No. 5,427,527 (Niznick) discloses other prior-art screw type implants. In for example FIG. 1B (showing prior art in relation to the invention of the referenced patent) a cylindrical implant is depicted having a smooth upper portion with an external hexagonal projecting head. The head portion is extending in a radial direction but only to a diameter having about the same size as the outer diameter of the threads of the shaft portion of the implant. Thus, this implant, although it is provided with some kind of head portion, seems to be prone to the same risk of fracturing the threads in the bore hole as the implant in WO 00/03657 (Astra Aktiebolag, Hansson) mentioned above. In the same document, a conical implant is described (FIG. 2). The angle of taper of the implant is preferably between 1° and 3°, and it is mentioned in the text that at least 50% of the implant length should be conical. The implant is to be inserted into a cylindrical bore hole in the bone tissue, being larger in diameter than a lower part of the implant and smaller in diameter than an upper part of the implant. Thus, when the conical implant is screwed into the cylindrical bore hole, the bone is spread about the upper part of the implant, which is supposed to increase the amount of bone to which the implant have contact.
In U.S. Pat. No. 5,427,527 (Niznick), the aim is to provide a conical implant to be used where there is little or so thin bone that narrower implants have previously been used. However, also in this prior art, the surgeon could easily continue to apply a growing torque even after the implant has reached the end of the hole, thus turning it so as to destroy the previously cut threads of the bore hole. Accordingly, this conical implant may be prone to the same problem as described above in relation to the implant of WO 00/03657 (Astra Aktiebolag, Hansson)
Another disadvantage is that the conical shape of the implant causes a large amount of bone to spread and consequently be subjected to extra loading. The idea is that this should increase the stability of the implant. However, the extra loading might over-load the bone tissue, resulting instead in bone resorption and following poor stability of the implant.